The compact cylindrical ceramic heater comprising a resistance heating element of high-melting metal as embedded between a core and an insulation layer covering the core is in widespread use as a heating means for the automotive oxygen sensor, glow system, etc. or as a heat source for devices for gassification of petroleum oil, such as a heater for use in semiconductor heating or an oil fan heater.
FIG. 6(a) is a perspective view showing a typical ceramic heater of this type schematically and FIG. 6(b) is a sectional view taken along the line A--A of Fig. (a).
This ceramic heater comprises a cylindrical core 31, an insulation layer 32 wrapping around said core 31 with an adhesive layer 37 interposed, and a resistance heating element 33 embedded between said core and insulation layer, with terminal portions of said resistance heating element 33 being connected to external terminals 34 disposed externally of said insulation layer 32 and lead wires 36 being connected to said external terminals 34, respectively.
As shown in FIG. 6(b), each terminal portion of said resistance heating element 33 and the corresponding external terminal 34 are interconnected via a plated-through hole 35 provided in the insulation layer 32 beneath the external terminal 34. In this arrangement, as an electric current is applied between the external terminals 34 through the lead wires 36, the resistance heating element 33 generates heat and thereby functions as a heater.
The insulation layer 32 of said ceramic heater generally comprises Al.sub.2 O.sub.3 supplemented with, as sintering aids, SiO.sub.2, MgO, CaO, etc. and, for such insulation layer, compaction to the theoretical density is difficult at the usual sintering temperature. Moreover, depending on characteristics of the starting material Al.sub.2 O.sub.3 powder, the particle size distribution of said sintering aids, and impurities, the ultimate density is sometimes more or less lower than a set density value.
In addition, as heating is continued for a long time, the alumina ceramics forming the insulation layer 32 is degraded by grain boundary migration etc. to develop voids in some cases.
In such cases, the resistance heating element 33 embedded in the alumina ceramics is oxidized to suffer a progressive increase in resistance and the resistance heating element 33 as such undergoes expansion at times. Moreover, as the oxidation of the resistance heating element 33 progresses, its heating temperature is altered and, in addition, the element 33 becomes easily destroyed and, in extreme cases, develop a disconnection trouble.
Moreover, because the resistance heating element usually comprises a high-melting metal and the metal and the ceramics are widely different in the coefficient of thermal expansion, the repeated heating load on the ceramic heater induces cracks across the interface between the resistance heating element and the ceramics owing to the thermal stress, resulting in local destruction of the ceramic heater and a disconnection in the resistance heating element.